This invention relates to a method and apparatus for separating air.
The most important method commercially for separating air is by rectification. Typically, a so-called "double rectification column" comprising a higher pressure and a lower pressure rectification column is used. Downstream of its being purified to remove components of low volatility such as water vapour and carbon dioxide, and being cooled to a temperature suitable for separation by rectification, most if not all of the air to be separated is introduced into the higher pressure column and is separated into oxygen-enriched liquid air and nitrogen vapour. The nitrogen vapour is condensed. A part of the condensate is used as liquid reflux in the higher pressure column. Oxygen-enriched liquid is withdrawn from the bottom of the higher pressure column is sub-cooled, and is introduced into an intermediate region of the lower pressure rectification column through a throttling valve. This oxygen-enriched liquid is separated into oxygen and nitrogen products in the lower pressure rectification column. Liquid nitrogen reflux for the lower pressure rectification column is provided by taking the remainder of the condensate from the higher pressure column, sub-cooling it, and passing it into the top of the lower pressure rectification column through a throttling valve.
It has been proposed that in a Heylandt cycle a `triple column` arrangement can be used with a third column operating at a higher pressure than the conventional higher pressure column of a double column arrangement. The third column has a condenser that provides reboil for the higher pressure column and the incoming air is admitted to the third column. The purpose of this arrangement is to remedy a shortage of liquid nitrogen reflux in the rectification columns of a Heylandt cycle when the oxygen product is withdrawn entirely as liquid from the lower pressure column, no products being withdrawn from the higher pressure column.
Conventionally, in a double column arrangement the lower pressure rectification column is operated at pressures in the range of 1 to 1.5 bar absolute so as to produce oxygen and nitrogen products at about atmospheric pressures. There is however today an increasing demand for producing oxygen at high rates at pressures well in excess of 2 bar for use in such processes as coal gasification. It is known from for example U.S. Pat. No. 4,224,045 that a relatively low purity oxygen product can be produced in the lower pressure rectification column when that column is operated at pressures well in excess of 2 bar. The need to compress the oxygen product is therefore reduced or obviated. A nitrogen product is also produced from the lower pressure rectification column. In order to recover work from the waste nitrogen stream, it is compressed and then expanded in the expander of a gas turbine. The nitrogen may be supplied to the combustion chamber of the gas turbine where it serves to reduce the formation of oxides of nitrogen.
In some instances, the expander has insufficient capacity to accept all the nitrogen product of the air separation plant. In such instances, an additional expansion turbine, independent of the gas turbine, may be used to recover work from the residual nitrogen product.
It is an aim of the present invention to provide a method and apparatus which makes it possible to produce gaseous oxygen and gaseous nitrogen products at pressure without the rate of production of such gaseous nitrogen being determined by the rate of production of such oxygen product.